1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly to a liquid crystal display device and a method of fabricating a liquid crystal display device.
2. Description of the Related Art
In general, liquid crystal display (LCD) devices control light transmittance through a liquid crystal material by application of an electric field. The LCD devices include a liquid crystal display panel having liquid crystal cells arranged in a matrix configuration, and a drive circuit to drive the liquid crystal display panel. Pixel electrodes and common electrodes are provided in the liquid crystal display panel to supply the electric field to each of the liquid crystal cells. The pixel electrode is formed on a lower substrate and the common electrode is formed on an entire surface of an upper substrate, wherein each pixel electrode is connected to a thin film transistor (TFT) to be used as a switching device. Accordingly, the pixel electrode drives the liquid crystal cell together with the common electrode in accordance with data signals supplied through the thin film transistor.
Fabrication of the lower substrate of the LCD device requires a plurality of mask and semiconductor processes, which are major factors in fabricating costs of the liquid crystal display panel. To solve this, a fabrication method for the lower substrate has a reduced number of mask processes. For example, one mask process includes several different processes, such as deposition, cleaning, photolithography, etching, exfoliation, and testing.
FIG. 1 is a plan view of a lower substrate of a liquid crystal display according to the related art, and FIG. 2 is a cross sectional view along II-II′ of FIG. 1 according to the related art. In FIG. 1, a lower substrate 1 (in FIG. 2) includes a TFT 30 located at each intersection part of the data lines 4 and the gate lines 2, and a pixel electrode 22 connected to the drain electrode 10 of the TFT 30.
In FIGS. 1 and 2, the TFT 30 includes a gate electrode 6 connected to the gate line 2, a source electrode 8 connected to the data line 4, and a drain electrode 10 connected to the pixel electrode 22 through a drain contact hole 20. In addition, the TFT 30 includes semiconductor layers 14 and 16 to form a conductive channel between the source and drain electrodes 8 and 10 by a gate voltage supplied to the gate electrode 6. Accordingly, the TFT 30 selectively supplies a data signal from the data line 4 to the pixel electrode 22 in response to a gate signal from the gate line 2.
The pixel electrode 22 is located at a cell area divided by the data line 4 and the gate line 2, and is formed of a transparent conductive material having high light transmittance. The pixel electrode 22 is formed on a protective layer 18 spread on an entire surface of the lower substrate 1, and is electrically connected to the drain electrode 10 through a drain contact hole 20 formed in the protective layer 18. A potential difference is generated between the pixel electrode 22 and a common electrode (not shown) formed in an upper substrate (not shown) by the data signal supplied through the TFT 30. The potential difference causes liquid crystal molecules located between the lower substrate 1 and the upper substrate (not shown) to rotate due to dielectric constant anisotropy. The rotated liquid crystal molecules allow transmission of light through the pixel electrode 22 from a light source toward the upper substrate (not shown).
FIGS. 3A to 3D are cross sectional views of a method of fabricating the lower substrate of FIG. 2 according to the related art. In FIG. 3A, the gate electrode 6 and the gate line 2 are formed on the lower substrate 1. For example, a gate metal layer including aluminum or an aluminum alloy is deposited on the lower substrate 1 by a deposition method, such as sputtering. Then, the gate metal layer is patterned by photolithographic and etching processes using a first mask to form the gate electrode 6 and the gate line 2 on the lower substrate 1.
In FIG. 3B, a gate insulating film 12, an active layer 14, an ohmic contact layer 16, a data line 4 (in FIG. 1), a source electrode 8, and a drain electrode 10 are formed on the lower substrate 1. For example, the gate insulating film 12, first and second semiconductor layers, and a data metal layer are sequentially formed on the lower substrate 1 by a deposition method, such as chemical vapor deposition or sputtering. The gate insulating film 12 is formed of an inorganic insulating material, such as silicon oxide SiOx or silicon nitride SiNx, the first semiconductor layer is formed of undoped amorphous silicon, the second semiconductor layer is formed of n-type or p-type amorphous silicon, and the data metal layer is formed of molybdenum Mo or an molybdenum alloy.
Next, a photoresist pattern is formed on the data metal layer using a second mask. For example, a halftone mask with a semi-transmitting part corresponding to a channel part of the TFT is used as the second mask, whereby the semi-transparent part of the photoresist pattern has a height lower than a height of the photoresist pattern corresponding to source/drain electrodes. Then, the data metal layer is patterned by a wet etching process using the photoresist pattern, whereby the data line 4 and the source and drain electrodes 8 and 10 are formed. Finally, the first and second semiconductors are simultaneously patterned by a dry etching process using the photoresist pattern to form an active layer 14 and an ohmic contact layer 16.
Next, the semi-transparent part of the photoresist pattern is removed by an ashing process, and the source/drain pattern and the ohmic contact layer corresponding to the channel part are etched by the etching process and the dry etching process. Accordingly, the active layer of the channel part is exposed to separate the source and drain electrodes 8 and 10. Then, the remaining photoresist pattern is removed from the source and drain electrodes 8 and 10 by a stripping process.
In FIG. 3C, a protective film 18 is formed over an entire surface of the lower substrate 1, wherein a drain contact hole 20 is formed to expose a portion of the drain electrode 10. For example, an insulating material formed of an inorganic insulating material, such as silicon oxide SiOx and silicon nitride SiNx, or an organic insulating material, such as acrylic organic compound, benzocyclobutene BCB, and perfluorocyclobutane PFCB, is deposited on the gate insulating film 12 provided with the source electrode 8, the drain electrode 10 and the data line to form the protective film 18. Subsequently, the protective film 18 is patterned by photolithographic and etching processes using a third mask to form the drain contact hole 20.
In FIG. 3D, a pixel electrode 22 is formed on the protective film 18 by depositing a transparent metal layer, such as indium-tin-oxide ITO, indium-zinc-oxide IZO, or indium-tin-zinc-oxide ITZO, on the protective film 18. Subsequently, the transparent metal layer is patterned by photolithographic and etching processes using a fourth mask to form the pixel electrode 22. Accordingly, the pixel electrode 22 is connected to the drain electrode 10 through the drain contact hole 20 formed in the protective film 18.
FIG. 4 is a cross sectional view of area P1 of FIG. 1 according to the related art. In FIG. 4, the data line 4 and the drain electrode 10 are formed with a specific gap therebetween at an area corresponding to an end projected part of the gate electrode 6. Accordingly, a short circuit often occurs due to a pattern defect between an active layer 14A formed at a lower part of the data line 4 and an active layer 14B formed at a lower part of the drain electrode 10 in an area except the gate electrode. Thus, a channel is formed due to the short circuit and receives light generated by a backlight device, wherein optical pumping current increases within the active layer 14. In addition, a voltage charged in the pixel electrode 22 (in FIG. 1) is discharged to the data line 4 through the channel, and a bright spot is generated since the voltage charged in the pixel electrode 22 becomes lower relatively.
FIG. 5 a cross sectional view along V-V′ of FIG. 1 according to the related art. In FIG. 5, since the gate electrode 6 cannot sufficiently cover the active layer 14 formed at the lower part of the source electrode 8, the active layer 14 receives the light generated by the backlight device to further increase the optical pumping current within the active layer 14. Accordingly, OFF-current of the TFT 30 increases.